What if the img tag supported only SVG Tiny, with the object tag required to use full SVG? This seems to (at least theoretically) answer the security concerns, while still allowing a well-documented subset of SVG in the img tag. I can see only two clarifications which would need to be added: foreign namespaces are not recognized when included through the img tag, and svg:image would not support SVG (but would support, for example, JPEG and PNG, and possibly other bitmap formats) when included through the img tag.
SVG Tiny doesn't support scripting, it doesn't necessarily have to support elements from other namespaces, and it doesn't necessarily have to support SVG in the SVG:image tag. Would an implementation of SVG Tiny which didn't support these two optional things be safe? Obviously you couldn't do everything with this that you could with full SVG, but I'm not sure this is a Bad Thing, since the object tag still exists to handle full SVG.
It seems to me that if this method can be shown safe then it would provide, if not necessarily the best of both worlds, then at least a decent compromise. The full power (and danger) of SVG would not be available through the img tag, but a well-documented standard subset would be. Tiny is, by itself, powerful enough to handle most aspects of vector graphics, and what it can't handle could be dealt with through the object tag.
It's just a thought, anyway. I might even be wrong about Tiny being safe, in which case the whole thing is moot. But if it would be safe, then might it be satisfactory?